The present invention relates to medical devices in general, and in particular to atherectomy devices for removing occluding material from a patient""s blood vessels.
Arteriosclerosis is a common vascular disease in which a patient""s blood vessels become hardened and blocked by plaque or clots that impede blood flow. Left untreated, this condition is a major contributing factor to the occurrence of high blood pressure, strokes and cardiac arrest.
To treat arteriosclerosis, many invasive and non-invasive techniques have been developed. For example, cardiac bypass surgery is now a commonly performed procedure whereby an occluded cardiac artery is bypassed with a segment of a healthy blood vessel that is obtained from elsewhere in the body. While this procedure is generally successful, it is fairly traumatic because the entire chest cavity must be opened to access the occluded vessel. Therefore, the procedure is not generally performed on elderly or relatively frail patients.
One example of a promising minimally invasive technique that can be performed on a greater number of patients is to remove the occluding material from a patient""s vessel in an atherectomy procedure. To perform this procedure, a guide catheter is typically inserted into the patient""s femoral artery and advanced until the distal end of the guide catheter is located in the patient""s ostium. A guide wire is then inserted through the guide catheter and traversed into the coronary arteries and past the occluded material to be treated. Then, as described in U.S. Pat. No. 4,990,134, issued to Auth, an atherectomy catheter having a small abrasive burr is advanced through the guide catheter and over the guide wire to the point of the occlusion. The burr is then rotated at high speed and passed through the occlusion to remove particles that are sufficiently small such that they will not reembolize in the distal vasculature. As the burr removes the occlusion, a larger lumen is created in the vessel and blood flow is restored.
It is well recognized that the risk of certain patient complications increases with the size of the guide catheter through which minimally invasive devices are routed. Larger guide catheters require larger access holes in the femoral artery, creating the potential for patient complications, such as the sealing of the puncture site after completion of the procedure. Therefore, physicians generally wish to utilize the smallest possible guide catheter during a procedure. However, the smaller size guide catheters can only accommodate corresponding smaller size ablation burrs. Therefore, if a large vessel is to be treated, a larger burr and corresponding larger guide catheter must be used to successfully remove all of the occlusion from the patient""s vessel.
In addition, it has also been discovered that when performing an atherectomy procedure as described earlier, it has been beneficial to remove only a small amount of the occlusion at a time. Therefore, currently many procedures are performed using multiple passes through the occlusion with different sized ablation burrs. While these procedures have proven effective, the use of multiple devices for a single procedure adds both time and cost to the procedure.
Given the disadvantages of the existing atherectomy devices, there is a need for an atherectomy device that can treat different size vessels while being traversed through a small guide catheter.
To eliminate the need for a physician to utilize larger guide catheters in order to route a larger diameter ablation burr in a patient, the present invention comprises an expandable ablation burr. The ablated diameter preferably has a diameter that exceeds the diameter of a guide catheter through which the burr is routed.
According to one embodiment of the invention, the ablation burr includes a polymeric balloon that expands as the burr is rotated. A portion of the balloon is coated with an abrasive such that the balloon will ablate an occlusion as the burr is rotated and advanced through a vessel.
In another embodiment of the invention, the expandable burr comprises a generally solid core with a nose section having a fixed, maximum outer diameter and a stepped proximal section with a smaller outer diameter. Positioned over the stepped section is a polymeric tube that is coated with an abrasive material. As the burr is rotated, the elastomeric tube expands by centrifugal force, thereby increasing the maximum outer diameter of the burr in order to create a larger lumen in a patient""s vessel.
In yet another embodiment of the invention, the ablation burr comprises a mandrel that is secured to a drive shaft. A metallic strip surrounds the mandrel. At least a portion of the metallic strip and mandrel is covered with an abrasive. When the metallic strip is tightly coiled around the mandrel, its outer diameter decreases. When released, the metallic strip will expand to the original outer diameter of the burr.
In yet another embodiment of the invention, the ablation burr includes a wire spring that is wound over a drive tube. A portion of the wire spring is coated with an abrasive material to ablate an occlusion in a patient""s vessel as the burr is rotated. A distal end of the spring is coupled to a nose cone that can move axially within the distal end of the drive tube. As the burr is rotated, the nose cone is drawn into the lumen. The maximum outer diameter of the burr is limited by the distance that the nose cone can move within the drive tube.
According to another aspect of the present invention, an ablation burr includes an indexing mechanism which allows the outer diameter of the burr to be selectively adjusted to create varying sized lumens in the patient""s vessel. By selectively controlling the length of the burr, the compression of a series of cutting blades that are coupled to the distal and proximal ends of the burr is changed in order to vary the outer diameter of the burr.
In one embodiment, the indexing mechanism includes a tube having a drive tube slidably secured to the proximal end thereof. The drive tube includes a fixed washer disposed at its distal end. The washer includes a number of teeth positioned around a distal rim. Disposed at the distal end of the tube is an indexing ring having a series of slots that encircle the indexing ring. Each slot has a different depth. A slide washer having a set of teeth that engage the teeth on the fixed washer is positioned over the indexing ring and tube. The slide washer includes a pin that engages a canted edge of the slots as the burr is rotated. The maximum distance that the drive tube can move with respect to a distal end of the burr is limited by the depth of the slot in which the pin on the slide washer is located. By controlling the movement of the drive tube with respect to the distal end of the burr, the maximum outer diameter of the cutting blades is controlled. As the blades are compressed by retrieving the burr into a catheter, the pin on the slide washer is moved to the next slot on the indexing ring such that the outer diameter of the burr can be varied.
In another embodiment of the invention, the indexing mechanism includes a drive tube having a race that extends around the perimeter of the drive tube along an axis that is canted with respect to its longitudinal axis. A traveling ball fits within the race. The drive tube also includes a series of ratchet teeth that extend around the perimeter of drive tube. Positioned over the drive tube is a proximal locking tube having a hole through which the traveling ball extends. Slidably aligned with the proximal locking tube is a distal locking tube that is coupled to the distal end of the ablation burr. Positioned over the proximal and distal locking tubes is a traveling tube having a hole in which a portion of the traveling ball is seated. As the drive tube is rotated with respect to the cutting blades, the traveling ball moves in the race thereby moving the traveling tube along the length of the drive tube and limiting the distance by which the distal end of the burr can move with respect to the proximal end of the burr and hence changing the maximum outer diameter of the cutting blades.
In yet another embodiment of the invention, the indexing mechanism includes a drive tube having a serpentine channel disposed around the perimeter of the drive tube. A proximal locking tube is slidably affixed over the proximal end of the drive tube. A distal locking tube is slidably aligned with the drive tube. The distal locking tube engages a distal end of the ablation burr. Positioned over the drive tube is a traveling tube having a pin that operates as a cam within the serpentine channel. As the pin moves within the channel, the traveling tube limits the movement of the distal locking tube with respect to the drive tube and hence limits the maximum outer diameter of the cutting blades.